1. Field of the Invention
The present invention relates to an inverter used for operating loads such as a three-phase induction motor, etc.
2. Description of the Related Art
Generally, an inverter is used for controlling the operation of loads such as an induction motor, etc. Said inverter converts an alternating current (hereinafter referred to as, “AC”) power into a direct current (hereinafter, “DC”) power by having a power supply unit. Then, said DC power operates the load by being switched to a plurality of switching elements by a switching unit and being converted into the AC power. An Insulated Gate Bipolar Transistor (IGBT) is widely used as said plurality of switching elements.
In such inverter, a torque and a rotating speed of the load are precisely controlled by controlling the switching time of said plurality of switching elements and varying the voltage and the frequency of the AC power supplied to the load.
When operating the load by the motion of said inverter, an over current supplied to said load may cause a fault of the load. Thus, the inverter has a fault detecting unit and detects whether the fault that the over current is supplied to the load is generated or not. If the fault detecting unit detects the generation of the fault, the switching elements of the inverter is stopped and prevents the load from being damaged by the over current.
Said fault detecting unit conventionally converts the current flowing to the load into the voltage at a current detecting resistance and detects the power flowing to the load by both ends of the voltage of the current detecting resistance by having the current detecting resistance in series between a power supply unit and a switching unit. Further, the fault detecting unit detects whether or not the fault for supplying the over current to the load is generated by comparing both ends of the voltage of said current detecting resistance with the predetermined standard voltage.
When the resistance value of said current detecting resistor is high, the power consumption in the current detecting resistance becomes large. Thus, said current detecting resistance is conventionally designed to have a low resistance value for reducing the power consumption at the current detecting resistor. Further, the generation of said fault is conventionally detected by using the difference between the voltage detected at both ends of the current detecting resistance when the normal current flows into the load and the voltage detected at both ends of the current detecting resistor when over current flows into the load.
By using the current detecting resistor having a low resistance value and detecting the generation of the fault by the voltage of both ends of the current detecting resistor, said switching unit may relatively exactly detect the generation of the fault when the capacity of the power supplied to the load is low.
However, when the capacity of a rated current supplied to the load by the switching unit is high, the switching unit cannot exactly detect the generation of the fault due to the power consumption generated at the current detecting resistor.
Therefore, a technique that can exactly detect the generation of fault of the switching elements without relating to the capacity of the rated current supplied to the load is necessary.